Check that the plugin doesn't stop pytest from running if Matplotlib isn't installed

Author: astrofrog

.travis.yml

@@ -20,6 +20,7 @@ env:
     - PYTHON_VERSION=3.6 TOXENV=py36-test-mpl22
     - PYTHON_VERSION=3.7 TOXENV=py37-test-mpl30
     - PYTHON_VERSION=3.8 TOXENV=py38-test-mpl31
+    - PYTHON_VERSION=3.8 TOXENV=py38-test-minimal
     - PYTHON_VERSION=3.8 TOXENV=codestyle
 
 install:

tox.ini

@@ -18,12 +18,14 @@ deps =
     mpl31: matplotlib==3.1.*
     mpldev: git+https://github.com/matplotlib/matplotlib.git#egg=matplotlib
 extras =
-    test
+    !minimal: test
 commands =
     pip freeze
     # Make sure the tests pass with and without --mpl
-    pytest {toxinidir}/tests --cov pytest_mpl --cov-config={toxinidir}/setup.cfg {posargs}
-    pytest {toxinidir}/tests --mpl --cov pytest_mpl --cov-config={toxinidir}/setup.cfg {posargs}
+    !minimal: pytest {toxinidir}/tests --cov pytest_mpl --cov-config={toxinidir}/setup.cfg {posargs}
+    !minimal: pytest {toxinidir}/tests --mpl --cov pytest_mpl --cov-config={toxinidir}/setup.cfg {posargs}
+    # Make sure that the plugin won't crash pytest if Matplotlib isn't installed
+    minimal: python -c 'import pytest_mpl.plugin'
 
 [testenv:codestyle]
 skip_install = true